When gypsum board panels (also known as “dry wall” in the industry) or the like are used as a wall surfacing in the construction of houses and other structures, joint compounds are typically used in combination with a paper or mesh tape material to close and fill the joints and corners between adjoining panels.
Joint compounds typically are provided and marketed in one of two forms—single component or ready mix which is a finished formulation ready to apply needing only stirring before application and two component setting dry mix which is in the form of a dry free-flowing powder to which water is added and stirred until a workable consistency is attained.
The two main components of joint compounds are a filler and a binder. Conventional fillers are inert such as calcium carbonate and calcium sulfate dihydrate (gypsum), and reactive such as calcium sulfate hemihydrate. The type of filler determines whether the joint compound is a setting or a drying compound.
A major portion of the filler in setting joint compounds is calcium sulfate hemihydrate. Optionally, small portions of inert fillers such as calcium carbonate, talc, mica, silica, clays, expanded perlite, calcium sulfate dihydrate, and other mineral fillers can be added to setting type joint compounds. The calcium sulfate hemihydrate takes up some of the water in the mix by hydration to form calcium sulfate dihydrate crystals when the compound sets. The balance of water evaporates or is absorbed into the substrate.
Calcium carbonate is normally the major portion of filler in drying type joint compounds. Optionally, small portions of inert fillers such as talc, mica, silica, clays, expanded perlite, and other mineral fillers can also be added to drying type joint compounds. Hardening occurs as the water is eliminated through evaporation or absorption into the substrate.
A binder serves to provide cohesive strength to the joint compound and to bond the compound to a surface. Examples of typical binders are poly(vinyl acetate), vinyl acetate copolymers, and poly(vinyl alcohol).
In a widely used technique for filling gypsum board joints, water is added to a dry mixture containing calcium sulfate hemihydrate to form a high viscosity joint compound paste which is applied in order to close the joint. A fiberglass, cloth, or paper taping material is then stretched over the joint and embedded in the soft joint compound. An overlayer or top dressing of the joint compound is applied over the taping material to completely fill the joint and provide a smooth surface. After setting, the top dressing can optionally be sanded to smooth out irregularities prior to painting or applying a wall covering, or other finishing material.
By adjusting the catalysts and retarders present in the joint compound the setting time can be adjusted to meet the requirements of the applicator.
One problem with conventional setting or drying type joint compounds is their friability. Cracks can develop if the joint is subject to movement either during cure or drying or after the joint compound has set or dried.
An application in which particular problems occur is factory manufactured houses or manufactured house modules. When components such as ceiling and walls are moved about during the construction process joint movement can occur and cracking can occur. Also, as the partially completed structure is moved through the various manufacturing steps the joints are subject to movement that can cause cracks. Additionally, as the completed structure is moved from the point of manufacture to another site the structure is subjected to bending, racking and twisting motion that can cause joints to flex and crack. The action of transport can result in excessive mechanical stress that causes cracks to form in the joints of the fabricated gypsum board surfaces.
An example of drying type compounds for flexible joints is described in U.S. Pat. No. 5,494,947. They contain an acrylate, a biocide, a reinforcing agent, a latex, surfactants, a plasticizer, coalescents, a fungicide and a filler. The compounds described in this patent are deficient in that the time to effect drying is long and unsuitable for the rapid assembly techniques used in factory assembled structures.
Traditional setting type compositions provide the rapid hardening needed for factory assembled structures but do not have sufficient flexibility and crack if the joint is subjected to movement.
Examples of compositions that combine emulsion polymers with one or more fillers are described below:
U.S. Pat. No. 3,297,601 (Maynard, et al., 1967) discloses a substantially dry joint compound containing calcium sulfate hemihydrate, poly(vinyl acetate) and starch which, when mixed with water, can be used for finishing joints. Low shrinkage and relatively fast set times are reported.
U.S. Pat. No. 3,947,398 (Williams, 1976) discloses a surfacing composition comprising gypsum plaster and an aqueous emulsion of polymer resin. The polymer resin is usually a polymer of ethylenically unsaturated monomers such as vinyl resins and polymers of acrylates and methacrylates. Styrene/acrylic copolymers are preferred. The resins desirably set to a resin phase which is hard at room temperature (65° F.) The proportion of dry resin solids is 10 to 35 wt %, based on the weight of dry plaster solids. Preferred compositions are reported to be ideally suited for domestic and industrial flooring.
U.S. Pat. No. 4,238,239 (Brown, 1980) discloses a dry wall joint compound containing a powdered mixture of 27 to 41 wt % gypsum, 24 to 38 wt % hydrated gypsum, 16 to 26 wt % calcium carbonate, 5 to 9 wt % plaster of paris, 4 to 6 wt % hydrated amorphous silica, and 1 to 3 wt % talc. The dry wall joint compound can be mixed with water or an aqueous mixture of 0.35 to 8 wt % zinc oxide, 0.35 to 1.5 wt % sodium acetate, 0.35 to 3 wt % ammonium alum, and 0.35 to 8 wt % of an acrylic resin. to form a dry wall joint cement. The joint cement is reported to be resistant to cracking and shrinkage.
The need remains in the industry for quick setting hydraulic joint setting compounds that prevents cracking of joints, especially during transport of prefabricated modules.